1. Field of the Invention
This invention relates to a process and a device for amplitude modulation of a radiofrequency signal.
It is applied in particular to medium wave radiobroadcast transmitters comprising power modules having field-effect transistors or, in English, MOSFET-type transistors.
2. Description of the Prior Art
With the advent of power transistors, easy to control and with short switching times therefore able to operate at frequencies from several hundred kilohertz to a megahertz or more, the costly, bulky electron-tube transmitters requiring very high voltage power supplies and of difficult maintenance tend to be replaced with transmitters whose amplifier stages use these types of transistors. These transmitters are generally composed of power modules or elementary amplifiers, assembled in parallel or in series so as to transmit the required power. The usual structure of these modules is of the "H"-bridge type also called "complete bridge." Currently, there are several solutions for amplitude modulation of the output signal of these transmitters. One is described by patent application No. 2,567,338 titled "Amplifier having amplifying stages that can be switched as a function of the amplitude of the signal to be amplified" filed in France in the name of the applicant,
This solution is relatively simple to use and does not require any special management of the operating of the elementary modules. Nevertheless, it requires a great deal of self inductances and capacitors designed for large voltages and circuits, as well as transformers with many secondaries, this is therefore a bulky costly solution of mediocre efficiency.
Another solution comprising a digital management of the modules makes possible an improvement in the efficiency but at the cost of a great complexity of managing the modules. In this case, the constitution of the envelope of the wave transmitted is performed by superposition of the signal provided by each of the power modules supplied with the same voltage E and whose operation is controlled by the digital part. However, to refine the envelope of the signal to be transmitted, it is necessary to use additional power modules supplied with voltage E/2, E/4 . . . E/2p. This constitutes a major drawback since all the elementary modules are no longer identical and thus eliminates the total interchangeability of these modules. This also does not make possible an operating mode at degraded or reduced power, since the disconnecting of at least one module, as a result of any failure, causes distortions of the transmitted wave in contrast with a solution composed of interchangeable modules whose stopping of one or the other causes only a reduction of emitted power, now, quite often in certain cases of figures, such an operating mode can be satisfactory for use.
Further, this solution requires many sources of different voltages and the continual switchings of the modules produce disturbing spurious signals. It should be noted moreover that the lack of total interchangeability of the modules causes additional difficulty and cost for maintenance and development.
The object of the invention is to eliminate said drawbacks.